The transmembrane chemokine fractalkine (CX3CL1) present on neurons and peripheral endothelial cells acts as an adhesion molecule or as a soluble chemoattractant. CX3CL1 signals through its receptor CX3CR1 which is expressed in microglia, monocytes/macrophages and dendritic cells, NK cells and T cells. Notably, multiple sclerosis patients revealed lower expression of CX3CR1 in peripheral NK cells when compared to healthy controls and lack of CX3CR1+ cells in peripheral blood correlated with disease activity. However, the role of CX3CR1 in antigen presenting cells and T cells, and their contribution to CNS pathology are still enigmatic. The hypothesis behind the proposed research is that CX3CR1/CX3CL1 regulates antigen presenting cell (APC) effector functions influencing the development of pathogenic T cells during experimental autoimmune encephalomyelitis (EAE). This hypothesis is based on the following: 1) EAE symptoms are more severe and CNS demyelination is enhanced CX3CR1-deficient mice, 2) absence of CX3CR1 correlated with a selective accumulation of CD115+CD11c+ dendritic cells to CNS tissues, and 3) Bone marrow chimeric mice revealed that absence of CX3CR1 in bone marrow induced an unusual, severe and chronic non-remitting EAE disease with sustained paralysis. The overall goal of this proposal is to elucidate the function of CX3CR1 during EAE and how it regulates pathogenic CNS inflammation. The specific aims are: 1. to determine the role of CX3CR1 for the initiation of EAE via modulation of antigen presentation and T cell priming. We will test the hypothesis that CX3CR1 controls peripherally DC maturation affecting antigen presentation and subsequently peripheral T cell polarization. We will investigate the role of CX3CR1 antigen trafficking and dendritic cell mobilization and effects of CX3CR1-deficiency in generation of encephalitogenic T cells. 2. To determine the role of CX3CR1 deficiency in the effector phase of EAE and protection from neuronal damage and demyelination. We hypothesize that absence of CX3CR1 in bone marrow is critical for the maintenance of T cell mediated inflammation and tissue damage in the EAE brain. We will investigate the role of CX3CR1 in inhibitory signaling, activation, and survival of pathogenic and regulatory T cell subsets within CNS tissues at peak of disease and at time of recovery. 3. To define address how weaker signaling through human CX3CR1I249/M280 replicates the pathology of Cx3cr1-/- mice during EAE. We hypothesize that I249/M280 expressing cells will exhibit effector functions comparable to CX3CR1-deficient cells. We will use knock-in mice expressing the human variant as a low affinity model to investigate the role of the I249/M280 in APC activation and T cell polarization in vivo during CNS autoimmunity.